(1) Field of the Invention
This invention generally relates to tape take-up reels, especially those having openings in their hubs for receiving tape leader blocks which are frequently used in magnetic tape drive systems. More particularly, this invention is concerned with automatically locating tape threader pins in the axis of rotation of these take-up reels so that they can freely rotate to wind and unwind the tape.
(2) Description of the Prior Art
Various devices have been developed for locating tape threader pins and their associated leader blocks in the hubs of tape take-up reels. For example, U.S. Pat. No. 4,350,309 teaches such location by virtue of the fact that the entire leader block is coextensive and otherwise mates with a slot in the hub of the take-up reel. This arrangement also allows the tape, which is attached to the tail end of the leader block, to be wound smoothly around the hub of the reel. Hence the leader block is held in place by the layers of tape which are wound around the hub. This winding action is provided by drive motors which are usually connected directly to take-up reels of this kind. Such motors must be reversible, have high speeds and be capable of quick starts and stops.
In such tape threader devices, location of the threader pin at the axis of rotation of the take-up reel depends upon four factors: an initial proper location of the threader pin within the leader block; the length of the leader block; the length of the hub's receiver slot; and the absence of any disturbances of the threader pin once it is properly located, e.g., a disturbance produced by "backlash" of the threader arm motor. Ideally the threader pin will be located at a position within the leader block which will correspond to the take-up reel's axis of rotation when the leader block is completely and properly located in the hub's receiver slot.
If the leader block is not completely in the receiver slot in the hub, three problems develop. In the first place, a bending force is placed upon the leader block and the threader pin as the reel tries to rotate and overcome the that fact that the threader pin is not centrally positioned on the reel's axis of rotation. In addition, a discontinuity is placed in the hub's surface by the protruding rear end of the leader block. This causes a "tenting" effect in the tape at the discontinuity. Moreover, a great deal of unnecessary vibration, noise and wear, particularly upon the leader block and base of the threader pin, is created when the threader pin's center does not coincide with the take-up reel's axis of rotation. On the other hand, if the front end of the leader block and/or the deepest end of the receiver slot become worn, the leader block will go too far into the receiver slot. The bad end results are the same. Bending forces are placed on the leader block and threader pin, discontinuities, due in this case to "detenting" effects upon the tape, develop on the tape windings and unnecessary vibrations, noise and wear are created.
Moreover, even if the leader block is initially located in the hub receiver slot in an ideal position for the threader pin, it can be easily disturbed during operation of the reel. The most common sources of disturbance are the quick starts and stops of the threader arm's motor. These quick starts and stops create a condition known as "motor backlash". That is to say the gears associated with the motor "recoils" from the forces placed upon them as they are called upon to wind in one direction and then to quickly wind in the opposite direction. Such motor backlash has a tendency to dislocate the threader pin and hence the leader block from its original proper position in the receiver. Any disturbance of the threader pin from an original proper position leads to the same results as an initial failure to attain a proper position--unwanted vibration, noise and wear.